fix: distinguish air vs solid non-visible tiles

Air (id=0) not in line of sight participates in depth fade (open space).
Solid terrain not visible is genuinely occluded — render black.

src/world/tiles/tilemap_chunk.rs

@@ -66,7 +66,7 @@ impl TilemapChunkSpawner {
 }
 
 pub const FIXTURE_ROW_OFFSET: u16 = 6;
-pub const LAYER_ALPHA: f32 = 50_f32 / 255_f32;
+pub const LAYER_ALPHA: f32 = 41_f32 / 255_f32;
 
 fn tile_id_to_tileset_index(tile_id: u32) -> Option<u16> {
     if tile_id == 0 {
@@ -157,14 +157,12 @@ fn populate_chunk_tiles(
                         let is_visible = is_tile_visible_at_z(t, z_index);
                         let tile_data = if is_visible {
                             tile_for_depth(tileset_idx, z_diff)
+                        } else if t.id == 0 {
+                            tile_for_depth(0, z_diff)
                         } else {
                             TileData {
                                 tileset_index: 0,
-                                color: Color::srgb(
-                                    133_f32 / 255_f32,
-                                    167_f32 / 255_f32,
-                                    178_f32 / 255_f32,
-                                ),
+                                color: Color::BLACK,
                                 visible: true,
                             }
                         };